This invention relates to a control device for driving or controlling the operation of a pulse motor, and more particularly to such control device which has a fail safe function of detecting abnormality in the pulse motor and taking necessary safe actions.
Conventionally, many control systems and driving systems have been proposed and actually used, which use pulse motors as actuators or the like.
For instance, an air/fuel ratio control system for controlling by means of feedback the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, to a predetermined value has been proposed by the assignee of the present application, which includes a valve connected to fuel quantity adjusting means such as a carburetor for controlling the air/fuel ratio of the mixture. The valve is controlled by a pulse motor used as the actuator for the valve, in response to a control signal outputted from an electronic control unit as a function of the concentration of an exhaust gas ingredient emitted from the engine.
The pulse motors used in many control systems and driving systems inclusive of the above air/fuel ratio control system are generally each comprised of a stator having a plurality of coils wound on its salient poles in a polyphase manner and a rotor formed with a plurality of salient poles, around which the stator is disposed. The coils wound on the stator are sequentially energized by means of an electronic control circuit, to cause each of the salient poles of the rotor to be magnetically attracted toward a corresponding one of the salient poles of the stator to thereby cause rotation of the rotor. In a pulse motor constructed above, if even a single one of the polyphase coils malfunctions due to disconnection fault in the coil per se or disconnection fault or short-circuit in the wiring system, or if a power transistor used in the electronic control circuit for energizing the coil becomes defective, there occurs a malfunction of the pulse motor such as stoppage and reversal. Particularly in the aforementioned air/fuel ratio control system, if no safety measure is taken in such an event, the air/fuel ratio of the mixture can be controlled to an improper value, which would have an unfavorable influence upon the driveability and exhaust emission characteristics of the engine.